In a time-of-flight mass spectrometer (which is hereinafter called the “TOF-MS”), a fixed amount of kinetic energy is imparted, with a predetermined period, to ions derived from a sample component, to make those ions fly a specific distance within a space and determine the mass-to-charge ratio of each ion from its time of flight. If there is a variation in the initial energy (initial speed) of the ions, a variation in the time of flight occurs among the ions having the same mass-to-charge ratio, which causes a deterioration in the mass-resolving power. To solve this problem, TOF-MSs which employ an orthogonal acceleration system (which may also be called “vertical acceleration” or “orthogonal extraction”) have been widely used (for example, see Patent Literature 1).
In the orthogonal acceleration TOF-MS, ions injected into an orthogonal accelerator section are given a fixed amount of energy with a predetermined period in a direction orthogonal to the direction of their injection, whereby a cluster of ions are ejected into a flight space. By accelerating the cluster of ions in a direction orthogonal to their incident direction, the orthogonal acceleration TOF-MS removes the influence of the variation in the time of flight due to the variation in the flight speed in the incident direction, and thereby improves the mass-resolving power.
In the orthogonal acceleration TOF-MS, a pair of electrodes are arranged within the orthogonal accelerator section in such a manner as to face each other across the area where ions are injected (orthogonal acceleration area). A pulsed voltage is applied to the pair of electrodes with the specific period mentioned earlier to eject ions into the flight space. The application of the pulsed voltage is performed, for example, by the switching of a voltage applied from a power source. The period with which the ions are ejected by the application of the pulsed voltage (which corresponds to the specific period mentioned earlier) is set so that the period will be longer than the time of flight of an ion having a mass-to-charge ratio which corresponds to the upper limit of the mass range to be measured in the TOF-MS.
The orthogonal acceleration TOF-MS is often used in combination with a liquid chromatograph or gas chromatograph. In such a chromatograph mass spectrometer, a plurality of target components temporally separated by the column of the chromatograph are introduced into the orthogonal acceleration TOF-MS and sequentially subjected to mass spectrometry. In this case, each target component generates different kinds of ions, and those ions have different mass-to-charge ratios. Therefore, the mass range to be measured is set for each target component, and the pulsed voltage is applied with a period corresponding that mass range to eject the ions into the flight space.